docs: def _save_cache(self, filename, section_number_of_pages, page_references):
          """Save the current state of the page references to `<filename>.rtc`"""
          cache_path = Path(filename).with_suffix(self.CACHE_EXTENSION)
          with cache_path.open('wb') as file:
              cache = (section_number_of_pages, page_references)
              pickle.dump(cache, file)